As semiconductor integrated circuits are designed in a fine structure and/or in a highly integrated manner, there is a demand for improving resolution (resolution power) of a projection exposure device that is used to manufacture such semiconductor integrated circuits. To meet such demand, a light source for exposure tends to have an even shorter wavelength. As a next generation light source for exposure of semiconductor, which comes after an excimer laser device, an extreme ultraviolet (EUV) light source device is studied. Such light source device can emit extreme ultraviolet light at a wavelength between 13 nm and 14 nm, particularly 13.5 nm. The EUV light source device is also used as a light source for inspecting (testing) a mask used for a projection exposure device that uses EUV light.
There are some known methods for the EUV light source device to generate (emit) the extreme ultraviolet light. One of the known methods heats an EUV radiation species (seed) for excitation. This generates a high temperature plasma. Then, the extreme ultraviolet (EUV) light is radiated and extracted from the high temperature plasma.
One kind of the EUV light source devices that employ such method is a discharge produced plasma (DPP) type EUV light source device. The DPP type EUV light source device utilizes the EUV radiation light from the high temperature plasma generated when the EUV light source device is driven with an electric current.
Li (lithium) and Sn (tin) draw attention as radiation species (seed) that are used by the EUV light source device to emit an EVU light at a wavelength of 13.5 nm and having a strong radiation intensity. In other words, Li and Sn draw attention as the high temperature plasma raw material for producing the EUV. The mechanism of the EUV radiation that relies upon the DPP method will be described briefly below.
According to the DPP method, electrodes are placed in, for example, a discharge vessel, and the discharge vessel is filled with a raw material gas (i.e., gaseous high temperature plasma raw material atmosphere). Then, discharge is caused to take place between the electrodes in the high temperature plasma raw material atmosphere so as to produce initial plasma. A self magnetic field results from an electric current that flows between the electrodes upon the discharging, and causes the initial plasma to shrink. As a result, the density of the initial plasma increases, and the plasma temperature steeply rises. This phenomenon is referred to as “pinch effect” hereinafter. Heating caused by the pinch effect elevates the plasma temperature. The ion density of the high (elevated) temperature plasma is 1017 to 1020 cm−3, and the electron temperature reaches approximately 20 to 30 eV. Then, the EUV light is emitted from the high temperature plasma.
In recent years, the DPP type EUV light source device uses solid or liquid Sn or solid or liquid Li. The solid or liquid Sn or Li is supplied to the surfaces of the electrodes, across which the discharge takes place, and irradiated with an energy beam such a laser beam for vaporization. Subsequently, the high temperature plasma is generated by the discharging. This is proposed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2007-505460). The following description deals with when the energy beam is a laser beam. This method is referred to as “LDP” method or “laser assisted gas discharge produced plasma” method in this specification.
Now, an LDP type EUV light source device disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2007-505460) will be described. FIG. 11 of the accompanying drawings shows a cross-sectional view of the EUV light source device disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2007-505460).
Reference numerals 114 and 116 designate disk-like electrodes. The electrodes 114 and 116 are disposed in a discharge space 112. The inner pressure of the discharge space 112 is regulated to a predetermined pressure. The electrodes 114 and 116 are spaced from each other by a predetermined distance, and rotate about rotation axes 146, respectively. Reference numeral 124 designates a high temperature plasma raw material 124 to emit EUV light at a wavelength of 13.5 nm. The high temperature plasma raw material 124 is a heated and melted metal, e.g., liquid tin, and is received in containers 126. The temperature of the melted metal 124 is regulated by a temperature adjusting unit 130 disposed in each of the containers 126.
The electrodes 114 and 116 are partially immersed in the melted metal 124 in the associated containers 126, respectively. The melted metal 124 that rides on the surface of each of the electrodes 114 and 116 is moved into the discharge space 112 upon rotation of the electrode 114, 116. The melted metal 124 which is conveyed into the discharge space 112, i.e., the melted metal 124 present on the surface of each of the electrodes 114 and 116 which are spaced from each other by the predetermined distance in the discharge space 112, is irradiated with the laser beam 120 emitted from a laser irradiation device (not shown). Upon irradiation with the laser beam 120, the melted metal 124 is vaporized.
While the melted metal 124 14 is being vaporized, a pulse electric power is applied to the electrodes 114 and 116. Thus, a pulse discharge is triggered in the discharge space 112, and a plasma 122 is produced. A large current is caused to flow upon the discharging. The large current heats and excites the plasma 112 such that the plasma temperature is elevated. As a result, the EUV radiation (EUV light) is generated from the high temperature plasma. The EUV radiation is taken out in the upper direction in the drawing.
Therefore, when the LDP method described in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2007-505460) is used, the solid or liquid target (high temperature plasma raw material) is irradiated with a laser beam, and the raw material is gasified (vaporized) to produce a gaseous high temperature plasma raw material atmosphere (initial plasma). Similar to the DPP method, the ion density in the initial plasma is, for example, approximately 1016 cm−3, and the electron temperature is, for example, approximately 1 eV or lower than 1 eV. Subsequently, the plasma temperature is elevated with the heating triggered by the discharge current drive. The ion density in this high temperature plasma becomes approximately 1017 cm−3 to 1020 cm−3, and the electron temperature becomes approximately 20 eV to 30 eV. As such, this high temperature plasma emits the EUV. Similar to the DPP method, therefore, the heating triggered by the discharge current drive in the LDP method, which is disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2007-505460), takes advantage of the pinch effect.
Reference numeral 148 designates a capacitor bank, which corresponds to a power source. The capacitor bank 148 is electrically connected to the melted metal 124, which is received in each of the containers 126, via insulated feed lines 150. Because the melted metal 124 is conductive, the electric energy is supplied to the electrodes 114 and 116, which are partly immersed in the melted metal 124, from the capacitor bank 148 via the melted metal 124.
According to this method, it is easy to vaporize tin or lithium, which is solid at room temperature, in the vicinity of the discharge region where the discharge takes place. Specifically, it is possible to efficiently feed the vaporized tin or lithium to the discharge region, and therefore it becomes possible to efficiently extract the EUV radiation at the wavelength of 13.5 nm after the discharging.
The EUV light source device disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. 2007-505460) has the following advantages because the electrodes are caused to rotate.
(i) It is possible to always feed a solid or liquid high temperature plasma raw material to the discharge region. The high temperature plasma raw material is a new EUV producing species (seeds).
(ii) Because that position on each electrode surface, which is irradiated with the laser beam, and the position of the high temperature plasma generation (position of the discharge part) always change, the thermal load on each electrode reduces, and therefore it is possible to reduce or prevent the wear of the electrodes.
An EUV light source device that uses another method is an LPP (Laser Produced Plasma) type EUV light source device. A mechanism for generating the EUV radiation on the basis of the LPP method will be described briefly below.
When the LPP method is used, a target is irradiated with a driver laser beam to produce the plasma. The material of the target is the high temperature plasma raw material that can generate the EUV. Similar to the LDP method, Li (lithium) and Sn (tin) draw attention as the material of the target. An EUV light source device that relies upon the LPP method, which is disclosed in Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 2007-529869), will be described below.
FIG. 12 of the accompanying drawings shows a conceptual view of the laser produced plasma EUV light source 220, which is illustrated in FIG. 1 of Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 2007-529869). The driver laser for generating the plasma may include a pulse laser system 222 (e.g., gas discharge excimer laser that is driven with high power and high pulse repeating frequency), a CO2 laser or a fluorine molecule laser.
The pulse laser system 222 is, for example, a gas discharge laser system that has a master oscillator power amplifier (MOPA) structure. The gas discharge laser system includes, for example, an oscillator laser system 244 and an amplifier laser system 248. The pulse laser system 222 has magnetic reactor switching type pulse compressing and timing circuits 250 and 252, and pulse power timing monitoring systems 254 and 256.
The light source 220 may also include a target conveying system 224 that conveys the target in the form of, for example, a droplet, a solid particle, or a solid particle contained in a droplet. The target may be conveyed into, for example, a chamber 226 by the target conveying system 224. The target may also be conveyed to an irradiation site 228, which is also known as an ignition site, by the target conveying system 224. Although not described in detail here, a system controller 260 conducts the control such that the target is irradiated with a laser pulse from the pulse laser system 222 along a laser beam axis 255 when the target is in a predetermined location.
The device disclosed in Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 2007-529869) detects a location of the target with, for example, droplet photographing (videotaping) devices 270, 272 and 274. Then, a feedback system 262 for detecting the target location is used to calculate the location and moving path (locus) of the target. Based on these pieces of information, the system controller 260 controls the position and direction of the laser beam. A target conveyance controlling system 290 corrects the releasing point of the target droplet 294, which is released by the target conveying mechanism 292, in response to a signal from the system controller 260.
In recent years, as described above, the target material is usually supplied in the form of droplet. The LPP method disclosed in Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 2007-529869) is believed to provide a light source that has power scalability and produces less debris (clean light source).
Such target conveying system needs to control the droplet size as well as, in terms of space and time, the laser beam irradiation position, and the feeding of the droplet. Such control is likely to be affected by disturbances, i.e., the temperature elevation caused by the plasma, and scattering of the target material and its residual upon the laser beam irradiation. The detail of such control system is described in Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 2007-529869).
It is said that the LPP method properly controls the target and creates less debris. However, it is recently said that even the LPP method exerts a large (unneglectable) influence on optical components when the LPP method intend to provide an output required from a semiconductor manufacturing process.
To date, the researches show that both of the LDP method and the LPP method use liquid tin (Sn) when a raw material for the high temperature plasma is supplied to generate EUV radiation. However, the LDP method has problems. Namely, scattering of tin and releasing of the electrode materials occur during vaporization of the raw material upon laser beam irradiation and discharge.
In the LPP method, which is considered to be a cleaner method than the LDP method, the tin scatters when the droplets are produced and the tin is irradiated with the laser beam. This produces a considerable amount of particles and ions, which deteriorate the life of the optical system. These particles and ions are called debris. When the output of the EUV radiation is enhanced, the debris is produced in a larger amount. This becomes a cause of significantly deteriorating the life of those optical components which are disposed downstream of the light emitting point.